In recent years, considerable advances have been made in the field of machine vision technology based upon optics. Techniques for mapping or imaging the surface of objects using such technology are well known. However, machine vision systems that rely upon optics are often unsuitable for the inspection of surfaces in situations where the roughness of the surface is of the order of the wavelength of the light used, or where there are large variations in surface reflective characteristics, or where contrast ratios are small. For example, plastic surfaces and dirty metal surfaces can be difficult to inspect.
In industrial environments, such difficulties may be compounded by fluctuations in background patterns or lighting conditions.
In the field of electrostatics, proximity sensors have been used in a wide range of applications for measurement of physical quantities. Such sensors are based on capacitance variation due to electrode displacement or due to local dielectric or conductivity variations. For example, three-electrode sensors with symmetrical clocks driving two parallel plates, and with a sensor electrode between, can measure small variations of position of the sensor electrode relative to the other two. As another example, gas pressure can vary the dielectric constant of a capacitor which is then inserted into a bridge circuit.
Further examples include elevator-type capacitance switches which may have a clock signal capacitively coupled to a detector, this coupling effectively being interrupted by a Faraday shield, whereby the contact of a finger shunts the signal to ground; level indicators for fluids which may make use of a capacitance varied according to the dielectric constant of, and the quantity of, fluid in a tank; and proximity fuzes for armaments which may make use of a change in capacitance caused by an externally applied object.
The primary object of the present invention is to provide new and improved apparatus for imaging a surface of an object located nearby, such apparatus being less sensitive to problems of the type outlined above for machine vision systems based upon optics.